1. Field of the Invention
The present invention relates to electrical connectors and more particularly to high I/O density connectors such as connectors that are attachable to a circuit substrate by use of a solder ball contact surface.
2. Brief Description of Earlier Developments
The drive to reduce the size of electronic equipment, particularly personal portable devices, and to add additional functions to such equipment has resulted in an ongoing drive for miniaturization of all components, especially electrical connectors. Efforts to miniaturize electrical connectors have included reductions in the pitch between terminals in single or double row linear connectors, so that a relatively high number of I/O or other lines can be interconnected by connectors that fit within the tightly circumscribed areas on circuit substrates that are allotted for receiving connectors. The drive for miniaturization has also been accompanied by a shift in manufacturing preference to surface mount techniques (SMT) for mounting components on circuit substrates.
The confluence of the increasing use of SMT and the required fine pitch of linear connectors has resulted in approaching the high volume, low cost limits of SMT for mounting connectors that employ presently available mounting designs. The limit is being reached because further reductions in pitch of the terminals greatly increase the risk of bridging adjacent solder pads or terminals during reflow of the solder paste. Array electrical connectors have been proposed to satisfy the need for increased I/O density. Such electrical connectors have a two dimensional array of terminal tails and can provide improved density. However, these connectors present certain difficulties with respect to attachment to the circuit substrate by SMT techniques because the surface mount tails of most, if not all, of the terminals must be attached beneath the connector body. As a result, the mounting techniques used must be highly reliable because of the difficulty in visually inspecting the solder connections and repairing them, if faulty.
Mounting techniques for other electronic components have addressed the reliability of solder connections in hard to inspect positions. For example, integrated circuit (IC) mounting to plastic or ceramic substrates have increasingly employed solder balls and other similar packages to provide such a reliable attachment. In a solder ball technique, spherical solder balls attached to the IC package are positioned on electrical contact pads of a circuit substrate to which a layer of solder paste has been applied, typically by use of a screen or mask. The unit is then heated to a temperature at which the solder paste and at least a portion of the solder ball melt and fuse to an underlying conductive pad formed on the circuit substrate. This heating process is commonly referred to as solder reflow. The IC is thereby connected to the substrate without need of external leads on the IC.
While the use of solder balls and similar systems in connecting ICs to a substrate has many advantages, a corresponding means for mounting an electrical connector or similar component on a circuit substrate has recently become desirable. The use of such techniques in mounting electrical connectors has lagged the use in mounting ICs because the use of solder ball technologies in mounting an electrical connector or similar component to a circuit substrate presents complexities not encountered with IC mounting. For example, ICs that have employed solder balls generally present a flat attachment surface. By contrast, connectors usually do not present a flat attachment surface but rather present a series of elongated conductors, commonly referred to as terminal tail ends. Attachment of a solder ball to the small end surface presented by the tip of a terminal tail end presents manufacturing difficulties not present in the attachment of solder balls to a flat surface.
In addition to the manufacturing difficulties, connectors are generally more susceptible to solder joint stress due to the effects of differential Coefficients of Thermal Expansion (CTE) between the connector and the circuit substrate. This susceptibility is primarily due to size and geometry differences between a connector and an IC. For example, IC mounting surfaces are generally on the order of 2.5 centimeters square. Connector mounting surfaces, on the other hand, generally have a narrow width (e.g., 0.5 centimeters or less)and a much longer length (e.g. 5.0 centimeters or more). Primarily because of the relatively long length of the connector, differences in CTE between a connector and a circuit substrate potentially have a much greater effect on the solder joints than the differences in CTE between an IC and a circuit substrate.
Connectors attached to circuit substrates via solder ball techniques are also more susceptible to joint stress than a conventional SMT attachment technique. For example, a conventional SMT attaches connector terminal tails to a circuit substrate horizontally, providing more attachment surface area for the solder joint. The additional surface area of the solder joint in the conventional SMT technique is stronger and, consequently, more tolerant of differences in CTE between the connector, terminal tails and circuit substrate. A solder ball connection, on the other hand, attaches a connector terminal tail vertically to the circuit substrate with the end of the terminal tail directly mated to the circuit substrate, reducing the amount of attachment surface area. As a result of the smaller attachment surface, differences in CTE are much more likely to stress the terminal tail to circuit substrate joint resulting in failure or quality problems.
Furthermore, in most circuit substrate applications, the electrical component mounting surfaces of the surface mount connections must meet strict coplanarity requirements. Thus, the use of solder balls to attach a connector to a circuit substrate imposes the requirement that the solder balls are coplanar in order to ensure a substantially flat mounting interface. So that, in the final application the balls will reflow and solder evenly to a planar circuit substrate. Any significant differences in solder coplanarity on a given mounting connection can cause poor soldering performance when the connector is reflowed onto a printed circuit board. Accordingly, users specify very tight coplanarity requirements to achieve high soldering reliability, on the order of 0.1 to 0.2 mm for example. By providing a connection using a solder ball technique, the coplanarity requirements can be met and sometimes exceeded. Unlike conventional SMT, the solder ball can absorb variations in terminal tail length by changing shape upon the application of heat to bridge the gaps between the terminal tail ends and the circuit substrate connections.
The present invention recognizes that there is a need for an improved electrical connector apparatus and accompanying electrical connector construction techniques that address the shortcomings of present electrical connectors.
The invention meets the above needs by providing an improved electrical connector for use in forming an electrical connection between a contact portion of an electrical component and a contact portion of a circuit substrate and method for constructing the electrical connector. The electrical connector comprises a connector body, a plurality of electrical contacts disposed on the connector body arranged to electrically mate with the contact portion of the electrical component, a plurality of electrical elongated conductors, alternately referred to as terminal tails, disposed on the connector body are arranged to form an electrical connection with the circuit substrate. The elongated conductors are in electrical communication with corresponding electrical contacts. A substrate contact, such as a solder ball, is connected via a butt joint on an end of each of the elongated conductors such that an electrical connection between the elongated conductors and the contact portion of the circuit substrate is selectively accommodated.
Each of the elongated conductors is disposed in a passage that has cross-sectional diameter a little larger than the cross-sectional diameter of the elongated conductor. As a result, clearance is provided between the sides of the electrical conductor and the side walls of the passage. Preferably, the cross-section is substantially rectangular in shape. The passages terminate in wells that are disposed across a planar face of the connector. The wells have a rectangular top opening that is longer along a length of the connector face. Moreover, a portion of the connector body proximate the elongated connector tail end is coated with an anti-migration solution such as oleophobic-hydrophobic flourochemical polymer to assist the process of solder ball formation and attachment.
One of the primary manufacturing challenges in manufacturing the above described connector involves the method of fusing substrate contact material (e.g., a solder ball) to the end of the tail portion of the elongated conductors. The invention accomplishes this attachment task by first forming a well, as described above, within a planar surface of the conductor. Here, the planar surface is provided by an interface member that can be formed separately and attached to the body of the connector or alternatively formed as an integral component with the body. In either case, the tail ends of the elongated conductors are inserted into passages formed in the interface member such that the tail ends terminate within a predefined range of the mounting surface and are exposed within the well. The well is then filled with a predetermined amount of solder paste. Finally, the substrate contacts are fused to the ends of the elongated conductors according to two embodiments.
In the first embodiment, a premanufactured substrate contact member such as a solder ball is seated into the paste. The tail end, substrate contact member and solder paste are then heated to a predefined temperature above the melting point of the paste such that the solder paste coalesces around the substrate contact member and joins it to the elongated conductor end.
According to a second embodiment, no premanufactured substrate contact member is used. Rather, a pre-specified amount of solder paste is applied to the well. Thereafter, the tail end and solder paste are heated to a predefined temperature, above the melting point of the solder paste. As a result, the solder paste coalesces into a ball attached to the end of the elongated conductor.
The process for forming a substrate contact on an elongated conductor as described above is further enhanced by coating the well with an anti-migration solution such as oleophobic-hydrophobic fluoropolymer. Thereafter, when the solder paste is heated, the paste is repelled from the treated surfaces of the well and interface member. This results in a more uniform ball formation. The substrate contact attachment process can be further enhanced by passivating a portion of the tail end such that solder paste will not attach to the passivated portion. As a result, the solder can attach only to the very end of the elongated conductor. Solder flow restriction at the tail end can be enhanced by passivating the tail end, coating the tail end with an anti-migration solution or both.